The present invention relates generally to a process for the rapid solidification of certain composite materials and to a composite product having an intermetallic containing matrix including an in-situ precipitated second phase, such as another intermetallic phase or a ceramic material, wherein the second phase comprises a boride, carbide, oxide, nitride, silicide, sulfide, etc., or intermetallic of one or more metals.
1. Field of the Invention
For the past several years, extensive research has been devoted to the development of metal-ceramic composites, such as aluminum reinforced with carbon, boron, silicon carbide, silica, or alumina fibers, whiskers, or particles. Metal-ceramic composites with good high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix. However, this technology has not extensively been used to produce composites having intermetallic matrices.
Intermetallics such as titanium aluminides are receiving increased attention for application as high performance structural materials. In particular, these ordered intermetallic compounds offer improved high temperature properties, including enhanced strength-to-weight ratios and oxidation resistance relative to conventional high temperature titanium alloys. However, general exploitation of these alloys has been limited by the lack of significant room temperature ductility and toughness, as well as the technical challenges associated with processing and/or machining the material into a final, usable form.
Compositing of titanium aluminides with particulate or fiber reinforcements creates the potential for additional improvements in alloy performance. For example, incorporation of a dispersed phase can result in direct strengthening of the matrix via dispersion or second-phase mechanisms, as well as stabilizing a fine matrix grain size. The latter can lead to additional improvements in processability (via enlargement of a stable processing "window"), as well as potential improvements in strength, ductility, and toughness.
Use of rapid solidification techniques creates the potential for additional alloying strategies, for example, the incorporation of rare earth alloying additions to produce a homogeneous, nanometer-scale rare earth oxide dispersion. Production of near-net-shape composite intermetallic components via powder metallurgy (P/M) techniques can minimize fabricability and machining problems inherent to intermetallic alloys.
2. Description of the Prior Art
Prior art techniques for the production of metal-ceramic composites may be broadly categorized as powder metallurgical approaches, molten metal techniques, and internal oxidation processes. The powder metallurgical type production of dispersion-strengthened composites would ideally be accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with the oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques or conventional procedures such as ball milling may be used to mix the powder. Standard powder metallurgy techniques are then employed to form the final composite. Conventionally, however, the ceramic component is large, i.e., greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials since their production is energy intensive, time consuming, and costly in capital equipment. Furthermore, production of very small particles inevitably leads to contamination of the particles with oxides, nitrides, and materials from various sources. Further, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature.
Alternatively, it is known that proprietary processes exist for the direct addition of appropriately coated ceramics to molten metals. Further, molten metal infiltration of a continuous ceramic skeleton has been used to produce composites. In most cases, elaborate particle coating techniques have been developed to protect the ceramic particles from the molten metal during admixture or molten metal infiltration, and to improve bonding between the metal and ceramic. Techniques such as these have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is only suitable for large particulate ceramics (e.g., greater than 1 micron) and whiskers, because of the high pressures involved for infiltration. The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the intersticies. Such a technique is illustrated in U.S. Pat. No. 4,444,603, of Yamatsuta et al, issued Apr. 24, 1984. Because of the necessity for coating techniques and molten metal handling equipment capable of generating extremely high pressures, molten metal infiltration has not been a practical process for making metal-ceramic composites.
The presence of oxygen in ball-milled powders used in prior art powder metallurgy techniques, or in molten metal infiltration, can result in oxide formation at the interface between the ceramic and the metal. The presence of such oxides will inhibit interfacial binding between the ceramic phase and the matrix, thus adversely effecting ductility of the composite. Such weakened interfacial contact can also result in reduced strength, loss of elongation, and facilitated crack propagation. In addition, the matrix may be adversely effected, as in the case of titanium, which is embrittled by interstitial oxygen.
Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as internally oxidized aluminum in copper. For example, when a copper alloy containing about 3 percent aluminum is placed in oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. This technique, although limited to relatively few systems since the two metals utilized must have a wide difference in chemical reactivity, has offered a feasible method for dispersion hardening. However, the highest possible level of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness, and the like. In addition, oxides are typically not wetted by the metal matrix, so that interfacial bonding is not optimum.
Because of the above-noted difficulties with conventional processes, the preparation of metal-ceramic composites with submicron ceramic dispersoids for commercial applications has been extremely expensive.
Another class of materials which has seen considerable interest and development is intermetallic materials, especially intermetallics of aluminum such as the aluminides of titanium, zirconium, iron, cobalt, and nickel.
The need for the advanced properties obtainable with intermetallic materials is typified by their potential application to structures capable of withstanding high temperatures, such as turbine engines. In designing and operating turbine engines today and for the foreseeable future, there are two primary problems which demand solutions from the field of materials science. The first of these is the need to operate certain portions of the engine at higher gas and metal temperatures to improve operating efficiency and save fuel. The second problem is the need for lighter materials to decrease engine weight and engine operating stresses due to heavy rotating components, and to increase the operating life of disks, shafts, and bearing support structures. These latter structures require materials which are less dense than the nickel base superalloys they are intended to replace, but which possess roughly the same mechanical properties and oxidation resistance as those materials in current usage.
The intermetallics are typically highly ordered compounds, in the sense that they possess regularly repeating (e.g., A B A B A B) atom sequencing. Intermetallic compounds are particularly suited to these needs because of two properties which derive from the fact that they possess ordered structures. Modulus retention at elevated temperature in these materials is particularly high because of strong A-B bonding. In addition, a number of high temperature properties which depend on diffusive mechanisms, such as creep, are improved because of the generally high activation energy required for self-diffusion in ordered alloys.
The formation of long range order in alloy systems also frequently produces a significant positive effect on mechanical properties, including elastic constants, strength, strain-hardening rates, and resistance to cyclic creep deformation. Finally, in the case of aluminides, the resistance to surface oxidation is particularly good because these materials contain a large reservoir of aluminum that is preferentially oxidized.
However, during metallurgical processing, one problem encountered is that these materials tend to form coarse grains, which degrade certain mechanical properties, the most important of which is ductility. Also, in many intermetallics the strong A-B bonding results in low temperature brittleness, although the exact mechanism of the ductile-brittle transition seems to be different for the different intermetallic compounds. It is thus necessary to address the problem of minimal low temperature ductility without destroying the inherent high temperature strength and stiffness. In the prior art it has generally been considered that these latter high temperature properties may only be retained by preserving the ordered structure, hence sacrificing low temperature ductility.
Since the early 1970's, the pace of work on ordered alloys and intermetallic compounds has slackened, as a result of lack of progress in improving either ductility or creep resistance of these otherwise very intriguing alloys.
Interest in utilizing ordered alloys for structural applications was reawakened in the United States when researchers discovered that ductility and strength improvements could be achieved in TiAl and Ti.sub.3 Al based alloys using a combination of powder metallurgy and alloying techniques. Later work on the titanium aluminides utilized ingot metallurgy. The development of rapid solidification methods led to renewed interest in the iron and nickel aluminides. The replacement of cobalt in Co.sub.3 V by nickel, and then iron, led to a series of face-centered cubic Ll.sub.2 -type superlattices with greater ductility at ambient temperatures. Also, it has been reported in Japan that polycrystalline Ni.sub.3 Al can be made more ductile by adding small quantities of boron. Later, this work was confirmed and the critical composition range over which boron was beneficial was identified. (See U.S. Pat. No. 4,478,791 of Huang et al, assigned to General Electric.) These discoveries, together with the national search for replacements for strategic metals, such as cobalt and chromium, and the need to develop energy-efficient systems, have in the past few years stimulated much additional works, largely in the area of improving low temperature ductility and increasing high temperature strength.
An example of work related to intermetallic matrix composites is taught in U.S. Pat. No. 4,774,052, of which this is a Continuation-In-Part. This patent application teaches a method for forming composite materials of descretely dispersed particulate second phase materials in intermetallic matrices, particularly in aluminide matrices. The dispersed material may constitute a second phase such as a ceramic, or an intermetallic compound other than the matrix metal.
Similarly, extensive research and development has been conducted in the area of rapid solidification (RS) processing. Rapid solidification processing effects highly desired forms of alloys. Homogeneous material at or above melt temperatures is subjected to a rapid quench or temperature drop to "freeze" the material to desired microstructure. The rate at which the melt is quenched is in the range of approximately 10.sup.4 .degree. C. per second to 10.sup.8 .degree. C. per second. See, for example, U.S. Pat. No. 4,402,745, hereby incorporated by reference.
Current technological interest in materials produced by RS processing, especially when used to produce fine powders followed by consolidation into bulk parts, may be traced, in part, to problems associated with the chemical segregation that occurs in complex, highly alloyed materials during conventional ingot casting and processing. During processing via slower cooling rates used for conventional casting processes, solute partitioning, that is, macro-and micro-segregation of different alloy phases present in these alloys, and the formation of undesirable, massive particle boundary eutectic phases, can occur. Metal powders produced directly from the melt by conventional powder production techniques, that is, drop tower, inert gas or water atomization of the melt, are usually cooled at rates three to four orders of magnitude lower than those that can be obtained by RS processing. The latter removes macro-segregation altogether and significantly reduces spacing over which micro-segregation occurs, if it occurs at all.
Design of alloys made by conventional slow cooling processes is primarily dictated by the corresponding equilibrium phase diagrams. Alloys prepared by such processes are in, or at least near, equilibrium. The advent of rapid quenching from a melt has enabled divergence from equilibrium and has added new alloys with unique structures and properties for commercial use.
Rapid quenching, or rapid solidification, techniques are known for manufacture of metal powder for powder metallurgical (PM) purposes by finely "atomizing" molten metal. Here, RS occurs not by contact but "in-flight". This technique permits little time for particle growth. The small drops produced solidify to form small granules, each one of which essentially constitutes an "ingot" of the molten metal. These small granules can be charged into a container that is evacuated and sealed. Afterwards, the small granules are compacted and concurrently or subsequently heated. This compaction and heating joins together the granules into a solid metal compact of the molten metal composition. This method is valuable for producing homogeneous materials from melt alloys which, if conventionally processed, would result in large-scale heterogeneities and segregation. Additionally, RS can produce materials containing fine metastable dispersoids and second phases.
Prior art techniques for "atomizing" molten metal have included impingement, melt spinning, and nozzle atomization.
In impingement techniques, atomization of molten metal into small drops is usually brought about in inert gas, such as argon or nitrogen. The gas impinges as high speed jets upon a pouring stream of molten metal. Water and steam have also been used. However, water and steam are unsuitable in certain instances because they cause severe oxidation of granules.
It is also known to atomize a pouring stream by impingement onto a rotating disk to make small drops or "ingots" which then solidify by contact with the surrounding atmosphere, cooling-water or oil bath, or a coolant shower. As mentioned above, in this approach the solidification does not occur by contact with the disk. That contact forms the drops or spheres which can have nearly monosize distribution.
British Patent Specification No. 519,624, hereby incorporated by reference, relates to powdered or granular metallic products constituted of solidified metallic particles derived from molten metal. It also describes a method of producing the product. These solidified metallic particles have spontaneously crystallized from a metastable undercooled state at a predetermined temperature below but close to the freezing point of the metal. The particles have substantially uniform size and composition
To produce such particles, molten metal is discharged from a suitable receptacle in one or more streams onto a metal surface of such nature that sufficient heat is abstracted from the molten metal to lower its temperature to that of an undercooled state, that is, to a temperature which is slightly below the freezing point of the particular metal but without causing solidification or crystallization. This surface upon which the molten metal impinges can be a belt or a disk rapidly moving either linearly or rotatively, respectively. The molten metal is immediately converted into a stream of film-like proportions on the surface and the extent of the belt or disk surface is such that the molten metal makes contacts therewith for a period just sufficient to undercool it as above defined. Then the molten metal is caused to leave the supporting surface and to continue its travel in the same direction and at substantially the same speed for a sufficient distance to cause solidification. Because the undercooled stream of film-like proportions has little or no integrity, it immediately breaks up into a myriad of fine, small liquid particles which solidify to form a powdered metal.
These operations may be carried out in a vacuum or suitable atmosphere, and the myriad of fine, small liquid particles may be passed through a coolant to hasten solidification of the particles or to reduce the distance needed for solidification. During solidification, surface tension causes the particles to assume a substantially spherical shape.
One known rapid solidification technique involving a centrifugal atomizing process is taught in U.S. Pat. Nos. 4,025,249 and 4,343,750, hereby incorporated by reference. It uses forced convective cooling of molten droplets to achieve cooling rates on the order of 10.sup.5 -10.sup.6 .degree. C./sec. This rapid solidification state is designated RSR. Such a RS technique, in conjunction with powder metallurgy techniques for consolidation of the rapidly solidified powders, has produced materials with metastable phases, very fine grain structures, high room-temperature strength and good high temperature properties up to the point of instability of the metastable phases.
An approach to further enhance certain material properties is to blend the RS powder with ceramic powders prior to consolidation. This leads to improvement in some mechanical properties, for example, modulus, hardness, etc. Silicon carbide aluminum (SiC/Al ), such as commercially available SiC/7090, produced by an RS/PM approach is an example of such a material. The difficulty with this approach is that it suffers from property and processing disadvantages inherent to a PM process. These difficulties include a relatively coarse reinforcement (greater than 1 micron) and/or weak metal/ceramic interfaces due to surface contaminants.
One alternative to conventional RS/PM techniques for developing dispersion strengthened alloys is to form the ceramic dispersoid phase during RS processing. U.S. Pat. No. 4,540,546, hereby incorporated by reference, describes a "Melt Mix Reaction" (MMR) process involving chemically reacting two starting alloys in a mixing nozzle in which a melt mix reaction takes place between the chemically reactable components of starting alloys to form submicron particles of the resulting compound in the final alloy. The mixing and chemical reaction is performed at a temperature which is at or above the highest liquidus temperature of the starting alloys but which is also substantially below the liquidus temperature of the final alloy, and as close to the solidus temperature of the final alloy as possible. While dispersion-strengthened alloys can be produced by this technique, there appear to be a number of inherent difficulties. First, processing is technically complex. Second, efficient mixing is important if fine dispersions are to be consistently produced. Lastly, very high degrees of superheat would be required to completely dissolve the RS alloying elements in order to produce high dispersoid loadings, which necessarily accentuate particle growth, for example, in one containing 10-20 % dispersoid.
In U.S. Pat. No. 4,240,824, hereby incorporated by reference, Moskowitz et al describe a process for producing a boron-containing nickel or cobalt spray-and-fuse self-fluxing alloy powder containing an internally precipitated chromium boride or nickel boride. In this patent, the starting materials are alloys containing precursors of the hard precipitate, and the melt is precooled to a temperature about 50.degree. F. higher than the viscous temperature prior to atomization. The particles are formed in the secondary atomization step, and are preferably larger than 10-15 microns in average particle size. No teaching is found for precipitating the particulate material prior to the atomization steps, or of precipitates having an average size less than 1 micron.
Narasimhan, in U.S. Pat. No. 4,268,564, hereby incorporated by reference, teaches the preparation of sheets or strips of amorphous metal containing embedded particulate matter, of 1 to 100 micron particle size, by forcing a glass-forming alloy containing particulate matter, formed in-situ, onto a rapidly moving chill surface. This technique was considered surprising because it had previously been believed that incorporation of particulate matter, especially of wettable particulate matter, into a molten glass-forming alloy would preclude quenching into an amorphous body due to nucleation of crystallization. Further, inclusion of particulate material in the metal melt in a melt spin process has led to rapid plugging of the orifice. The reference does not teach preparation of a rapidly solidified powder having an evenly dispersed particulate material therein. In fact, the reference specifically teaches that the particulate material is concentrated at the surface of the strip material produced.
These prior art techniques produce conventional powdered metal products.
U.S. Pat. No. 4,836,982, of which this application is a Continuation-In-Part, relates to an invention which overcomes the disadvantages of the prior art noted above, including current rapid solidification technology, and provides for the rapid solidification of composite materials comprising metal and metal alloy matrices.
The cited invention may also result in improvement from incorporation of a stable dispersoid into the composite which extends the high temperature working range of the composite, in contrast to conventional RS composites that typically contain metastable phases. Moreover, incorporation of dispersoids prior to RS may provide surfaces for precipitation, and consequently, a more efficient precipitation of metastable rapid solidification phases. In some cases, for example, titanium-based alloys, the addition of rare earth elements, like cerium or erbium, to the dispersoid-containing melt may result in improved scavenging of interstitials such as oxygen, leading to the formation of an additional oxide dispersoid and effective deoxidation of the matrix alloy.
It has now been found that one may form an intermetallic-second phase composite in accordance with the teachings of U.S. Pat. No. 4,774,052, and subject such material to rapid solidification in accord with the teachings of U.S. Pat. No. 4,836,982, to achieve a rapidly solidified intermetallic matrix of very fine grain size having a second phase dispersed therein.